(i) Field of the Invention
This invention relates to a polypropylene wax and its production process. Described specifically, the present invention is concerned with a polypropylene wax having a syndiotactic structure and also with a process for its production.
This invention also relates to an electrophotographic toner composition fixable by a heating roll, which toner composition uses as a releasing agent a polypropylene wax having a syndiotactic structure and is usable to develop a latent electrostatic image in electrophotography, electrostatic recording, electrostatic printing or the like.
(ii) Description of the Related Art
Low molecular-weight polypropylenes have conventionally been used as waxes in application fields such as pigment dispersants, rubber processing aids, resin processing aids, ink or dye additives, fiber treatment agents, and electrostatic copying toners. Isotactic polypropylene is now employed as a releasing agent especially for electrophotographic toner. There is, however, an outstanding demand for the development of a releasing agent suitable for use in faster copying.
Japanese Patent Laid-Open No. 41305/1990 discloses a polypropylene having relatively high syndiotacticity, which is obtained by polymerizing propylene in the presence of a catalyst system composed of a specific transition metal compound and a cocatalyst. The molecular weight of the syndiotactic polypropylene disposed in this patent publication is however so high that it is not suited as a wax.
Japanese Patent Laid-Open Nos. 274703/1990 and 274704/1990, on the other hand, disclose that the molecular weight of a syndiotactic polypropylene can be controlled by hydrogen upon production of the syndiotactic polypropylene in the presence of a catalyst system composed of a transition metal compound, which is similar to that disclosed in Japanese Patent Laid-Open No. 41305/1990 referred to above, and aluminoxane. The method disclosed in these patent publications has however been found difficult to provide a molecular weight suited for a wax.
Japanese Patent Laid-Open No. 197516/1991 discloses that an isotactic polypropylene wax can be produced by polymerizing or copolymerizing propylene in the presence of hydrogen while using a catalyst system composed of a particular metallocene and aluminoxane. The waxes disclosed in the patent publication however have a large enthalpy of melting. This patent publication also discloses a method for reducing the enthalpy of melting by increasing the content of a comonomer. This method is however accompanied by the drawback that the melting point of the wax also drops with the enthalpy of melting.
An isotactic polypropylene wax generally has a relatively high melting point as a merit, but a limitation is imposed on its utility due to its large enthalpy of melting. It has hence been desired to develop a polypropylene wax which has a relatively high melting point and a small enthalpy of melting.
In electrophotography, on the other hand, the copying speed tends to increase further in recent years to meet the ever increasing quantity of information to be dealt with. In the meantime, electrophotographic printers are finding ever-increasing utility as compact domestic copying machines and also printers for personal computers and office computers. Upon high-speed copying or printing, the quantity of heat which toner receives from a thermal fixing roll becomes smaller compared with low-speed copying or printing and, moreover, some heat is taken away by copying paper so that the surface temperature of the fixing roll is lowered considerably, leading to the problem that the fixing strength is insufficient. Such compact copying may chines and electrophotographic printers, however, structurally do not permit use of a heating roll having a large heat capacity, whereby insufficient fixing arises. With a view toward achieving fixing with a smaller heat quantity, it has been attempted to lower the melt viscosity of toner. This has however resulted in the occurrence of offsetting so that the above problem still remains unsolved. There is hence an outstanding desire for a toner which can be fixed by a heating roll of a low heat quantity and does not develop offsetting at the fixing temperature. A great deal of work has been made accordingly, including the addition of a wax as a binder and/or a releasing agent. Especially, with a view toward simplifying the maintenance of a machine and improving the writing property of fixed paper, there is a recent move toward adopting an oilless fixing heating roll instead of the conventional system in which silicone oil is coated on a surface of a heating roll to prevent offsetting. This has resulted in an ever-increasing demand for a further improvement in offset resistance so that the role of an offset preventive in toner has become important.
Some methods are known for improved offset resistance. As disclosed, for example, in U.S. Pat. No. 4,486,524, a resin having a weight average molecular weight/number average molecular weight ratio of 3.5 to 40 and a number average molecular weight of 2,000 to 30,000 is used as a binder to provide a toner having good offset resistance. Reissued U.S. Pat. No. 31,072 discloses use of a crosslinked binder resin to broaden the fixing temperature range, thereby providing a toner which does not develop offsetting even at a relatively high fixing temperature. Each of the above methods increases the viscosity of the binder resin upon melting so that the toner can be prevented from moving toward the fixing roll. Where the conduction of heat is insufficient as in high-speed machines, these methods cause a problem in fixing strength. It is also known, as disclosed in U.S. Pat. No. 4,921,771, to improve the offsetting problem by using an isotactic polypropylene having a number average molecular weight of 3,000 to 4,000 in an amount of 1 to 10 parts by weight per 100 parts by weight of a binder resin. In a fixing system where heat is not conducted sufficiently, however, the isotactic polypropylene wax so added cannot melt so that its offset preventing effect is reduced. If the amount of the wax is increased to compensate for the melting difficulty, the particle size of the wax dispersed in the toner becomes greater and, in some worst cases, wax particles are separated from toner particles. This causes problems in the quality of pictures, such as scattering and fogging due to an abnormal charge distribution, and also the so-called filming that the wax remains on a surface of a photoconductor. This filming leads to the trouble that images can no longer be formed there.
Among polyolefins employed as wax-type releasing agents, polypropylene is considered to have greater parting effects than polyethylene so that polypropylene is employed widely. These known polypropylene waxes mainly have an isotactic structure as disclosed in Japanese Patent Laid-Open No. 203409/1989. These isotactic polypropylenes generally have a large enthalpy of melting and have poor compatibility with binder resins for toner, so that they have not overcome the problem described above. According to the method disclosed in Japanese Patent Laid-Open NO. 197516/1991, ethylene blocks are introduced to improve the enthalpy of melting. The offset resistance is however deteriorated so that this method is not preferred. Japanese Patent Laid-Open No. 20509/1992 discloses a process for the production of a thermally degraded wax, while Japanese Patent Laid-Open No. 84009/1991 discloses a wax which is obtained by thermally degrading a propylene copolymer comprising a propylene and having an isotactic structure, ethylene and a C.sub.4 or higher .alpha.-olefin. Isotactic polypropylene is however not preferred because, when subjected to thermal degradation, it forms unnecessary sticky low molecular substances and toner blocking and filming are induced. To eliminate unnecessary sticky components formed by thermal degradation, Japanese Patent Laid-Open No. 121460/1991 discloses to obtain a high-isotactic polypropylene either by washing a low molecular-weight isotactic polypropylene, which has been obtained by thermal degradation, with toluene, or by thermally degrading a high molecular-weight polypropylene having high isotacticity. The enthalpy of melting however increases with the isotacticity so that such a high-isotactic propylene is not preferred for fixing toner on a heating roll with smaller energy and for preventing offsetting.
As has been described above, the conventional polypropylene waxes have a large enthalpy of melting and poor compatibility with other polymers. They are hence not suited for use in toners which can be fixed with smaller energy as required in recent years. Therefore, there is an outstanding demand for the development of a thermally fixing toner composition, which is excellent in offset resistance and permits low-energy fixing, by using a polypropylene wax having a small enthalpy of melting and good compatibility with other polymers.